Selective reactions involving gaseous reagents such as catalytic hydrogenation or oxygenation typically are conducted in either a flow system or a batch reactor. In a flow system the reaction can be run in the vapor phase or in the liquid phase, while in a batch process only liquid phase reactions are feasible. Vapor phase conditions are suitable for selective reactions, but are limited by the volatility and temperature stability of the reactants and products. Liquid phase reactions have a more general application.
In a selective liquid phase reaction such as the hydrogenation of a diene or acetylene to a monoene, the reactant is dissolved in a solvent and either stirred with the catalyst under an atmosphere of hydrogen (batch mode) or passed through a bed of catalyst while in contact with hydrogen gas. In either mode the reaction selectivity is influenced primarily by the nature of the catalyst. The desired monoene is produced because of a preferential adsorption on the catalyst of the reacting diene or acetylene which displaces the monoene before complete saturation can occur.
Hydrogen availability to the catalyst also affects this type of reaction selectivity in that the desorption of the monoene will be favored over its saturation when a limited amount of hydrogen is available to the catalyst. In both flow and batch reactors hydrogen availability is determined by the extent of gas/liquid diffusion of the hydrogen, and is controlled by varying the hydrogen pressure, rate of agitation, degree of hydrogen sparging, and the like. The degree of hydrogen availability control by these means is limited, so that invariably the hydrogen is present in large excess over the organic reactant substrate, and reaction selectivity is controlled primarily by the properties of the catalyst.
In many cases extensive preparation procedures are required to modify a catalyst in order to obtain the desired selectivity. This is illustrated by the selective hydrogenation of 4-vinylcyclohexene to 4-ethylcyclohexene with a nickel boride catalyst or a nickel arsenide catalyst prepared by the reduction of nickel arsenate with sodium borohydride in the presence of either silica or alumina as described in references such as J. Am. Chem. Soc., 85, 1005 (1963); J. Org. Chem., 38, 2226 (1973); and U.S. Pat. No. 4,716,256; 4,659,687; and 4,748,290. These catalysts generally provide about a 50% selectivity at 75-100% conversion. Treatment of an arsenide catalyst with ammonia increases the selectivity to 83% at 72% conversion. When the hydrogenation reaction is conducted in the presence of acetone over an alumina supported arsenide catalyst, a 96% selectivity at 96% conversion is obtained. Overreduction of an arsenide catalyst results in a reactivity which favors double bond isomerization. (J. Catal., 27, 397 (1972) reports that overreduced catalyst yields only about a 10% selectivity at 95% conversion, and the main product consists of double bond isomers.
There is continuing interest in chemical reaction systems for improved selective conversion of organic compounds.
Accordingly, it is an object of this invention to provide a flow liquid-phase chemical reaction system for selective conversion of organic compounds with a gas reactant.
It is a further object of this invention to provide a chemical reaction system with novel means for control of the molar ratio of gas reactant and organic compound dissolved in a solvent medium flowing through a catalyst bed.
Other objects and advantages of the present invention shall become apparent from the accompanying example and drawings.